Integrated circuits often contain analog functional unit blocks (FUB) that require one or more reference voltages. For example, in FIG. 1, die 102 comprises a microprocessor with many sub-blocks, such as, for example, phase-locked loop 116, voltage regulator 118, and interconnect driver 120. Some of these FUBs allow die 102 to communicate with other integrated circuits, such as off-die cache 104 and higher memory hierarchy levels, such as system memory 108 accessed via host bus 110 and chipset 112. The reference voltages provided to these various FUBs are often used to set some circuit property, such as, for example, amplifier bias, PLL frequency, or the output voltage of a voltage regulator. The reference voltage should be stable with respect to low and high frequency noise coupled through the semiconductor substrate of die 102, interconnects, or power supply 114. Otherwise, the performance of various FUBs may be degraded. For example, the output voltage of a voltage regulator may fluctuate if the reference voltage is not stable.
Bandgap circuits have been used to generate local reference voltages. However, a bandgap circuit makes use of an amplifier, which may add an offset as well as high frequency power supply noise to the reference voltage. Bandgap circuits at different locations on a die may not provide identical reference voltages due to variations in offset and noise power. A single bandgap circuit may be used to distribute a reference voltage as a single ended signal to different locations on a die. However, a single ended signal is not tolerant to noise coupled through a power supply or the substrate, for example. As a result, differential signaling is often preferred to single-ended signaling. A receiving FUB may utilize a received differential signal directly, or translate the differential voltage into a single-ended voltage referenced to a local ground or local VCC.
Ideally, common-mode noise in a differential signal may be cancelled by forming the difference of the differential signal to arrive at a single-ended signal. An example of a differencing (or subtracting) circuit is shown in FIG. 2. A differential signal is provided at input ports 202 and 204. Resistors 206 are designed to have the same resistance, and the voltage at output port 208 is referenced to ground 210 and is ideally the difference of the voltages at input ports 202 and 204. Common-mode noise at input ports 202 and 204 is ideally subtracted out. However, in practice there is a matching error in resistors 206, and their resistance (for well resistors, for example) may be temperature and voltage dependent. Furthermore, there may be an offset and high frequency noise coupling due to amplifier 212. A low-pass RC filter may be used to filter out high frequency power noise, but due to large MOS gate leakage in present day process technology, it has become difficult to implement an area efficient RC filter with a time constant larger than about 1 nanosecond. There is consequently a need for a noise tolerant voltage distribution technique for present day process technology.